Method for photo annealing non-single crystalline semiconductor films

ABSTRACT

An improved semiconductor processing is disclosed. In the manufacturing process, just formed semiconductor layer undergoes photo annealing and latent dangling bonds are let appear on the surface and gaps, then neutralizer is introduced to the ambience of the semiconductor. The semiconductor thus formed demonstrates SEL effect in place of Staebler-Wronski effect.

This application is a continuation of Ser. No. 074,344, filed 7/14/87,now abandoned, which itself was a Divisional Application of U.S. Ser.No. 891,791, filed Aug. 1, 1986, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an improved semiconductor manufacturing devicein which SEL(State Exited by Light) effect can be enjoyed.

2. Prior Art

There have been known processing techniques for semiconductor devicescomprising a substrate and nonmonocrystalline semiconductor layer formedon the substrate. In many products according to such techniques, someundesirable effects are observed. Namely, immediately after fablication,a highly purified semiconductor exhibits decrease of conductivity underphoto anealing in atmospheric air whereas it recovers by virtue ofthermal annealing. The effect appears repeatedly. The inventers havefound that such a phenomenon, called Staebler-Wronski effect is observedonly on semiconductors taken out from a vacuum chamber and made incontact with atmosphere.

Not only does repetition of increase and decrease in conductivity, occurbut the conductivity tends to gradually decrease as with repetition ofthe Staebler-Wronski effect. This is undesirable especially insemiconductors applied to solar cells.

SUMMARY OF THE INVENTION

An object of the invention is to produce improved semiconductors whichare thermally and optically stable.

Another object of the invention is to produce improved semiconductors onwhich is unlikely generation of dangling bonds.

Further object of the invention is to provide an improved semiconductorlayer which is not degraduated even under repetition of Staebler-Wronskieffect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a plasma vapour reactor as an embodimentof the invention.

FIG. 2 is a cross section view showing a method for measuring theelectric conductivity of a semiconductor according to the embodiment ofthe invention.

FIG. 3 is a graphical diagram showing the electric conductivitycharacteristic of a prior art intrinsic semiconductor.

FIGS. 4, 5, 6, 7 and 8 are graphical diagrams showing electricconductivity charactristics of semiconductors formed by a semiconductormanufacturing device according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an embodiment equipped with an ultra high vacuumapparatus according to the invention is shown. In the figure, asubstrate 10' made of artificial quartz is placed below a heater 12' ina first pre-stage chamber 1. The substrate 10 is provided with a pair ofelectrodes (designated as 24 and 24' in FIG. 2) for measuring electicconductivity. To this electrodes can be made contacts with a pair ofprobes 17 and 17' from outside after formation of a semiconductor layerfor making it possible to measure in situ the conductivity of thesemiconductor in the presence of light and the conductivity in theabsence of light, without making contact with atmospheric air.

The substrate 10' can be transported togather with the heater 12'between the first and second chambers 1 and 2 through the gate valve 3.The second pre-stage chamber 2 is provided with a criosorption pump 6through a second gate valve 5 and a turbo molecular pump 8 through athird gate valve 7. After placing the heater 12' on which the substrate10' is held in the first chamber 1, the turbo molecule pump 8 is drivento evacuate the interiors of the first and second chambers 1 and 2 withthe gate valves 3 and 7 opened and the gate valves 5 and 4 closed. Whena pressure of 10⁻⁶ torr or less has been attained in the chambers 1 and2, the heater 12' with the substrate 10' are transported from the firstchamber 1 to the second chamber 2 by means of a first transportationmechanism 19. Then the interior of the second chamber 2 is furtherevacuated to the order of 10⁻¹⁰ torr with the gate valves 3 and 7 closedand the gate valve 5 opened by means of the criosorption pump 6.

Upon making the interior of a reaction chamber 11 evacuated to anegative pressure of 10⁻⁹ to 10⁻¹⁰ torr by a second criosorption pump 9connected to the reaction chamber 11 through a gate valve 22, thesubstrate 10 on the heater 12 is transported from the second pre-stagechamber 2 to the reaction chamber 11 by means of a second transportationmechanism 19' through a gate valve 4 opened. Then the valve 4 is closedand between a pair of electrodes 14 and 15 are taken place plasmadischarge supplyed with power from a high frequency voltage supply 13 tocarry out a plasma CVD method on the substrate 10. Concurrent with theplasma discharge, light irradiation may be carried out by irradiatingthe interior of the reaction chamber 11 with an eximer laser or the likethrough a window 12.

In the figure, although two sets of substrates 10 and 10' and heater 12and 12' appear, this is made only to expedite explanation andunderstanding the embodiment and practically there is one set alone ofcourse.

The reactive gas for the plasma CVD method is introduced from a dopingsystem 21 to the interior of the reactive chamber 11. The pressure ofthe reaction chamber is kept suitable for carrying out the CVD methodoptimally by the turbo molecular pump 9.

The pressure in the reaction chamber 11 is kept at 0.001 to 0.1 torr,normally at 0.05 to 0.1 torr. By virtur of high frequency energy, e.g.,of 13.56 MHz and 10 watt, applied from the power supply 13, anonmonocrystalline semiconductor film, an amorphous silicon doped withhydrogen in this embodiment, is formed according to the plasma CVDprocessing. For example, a nonmonocrystalline semiconductor layer ofthickness 0.6μ which is doped or not doped with p-type or n-typeimpurities is formed on the substrate 10 at 250° C. (or less than 500°C.).

The reactive gas and a carrier gas should be purified to the level inwhich the inclusive rates of oxygen and water are reduced to less than0.1 ppm, more preferably to less than 1 ppb before introduction to thereactive chamber 11 in order to reduce the minimum oxygen density in thesemiconductor less than 5×10¹⁸ cm⁻³, more preferably less than 1×10¹⁸cm⁻³. As such a reactive gas, silane can be employed which is highlypurified by a liquefaction purification for fabrication of silicon film.

In case where a photoelectric cell is to be formed, highly doping issuitable. To make a p-type semiconductor, the silane gas is dosed withdiborane at 500 to 5000 ppm. To make a n-type semiconductor the silanegas is dosed with phosphin at about 5000 ppm. The impurities areintroduced from a inlet port 21".

Upon completion of the forming the semiconductor layer 26, the supply ofthe reactive gas is interrupted and the residue in the reaction chamber11 is eliminated.

Then oxygen, fluorine, chlorine or nitrogen is introduced as aneutralizer agent from the doping system 25 to the first prestagechamber 1.

After evacuating the reaction chamber 11 by the turbo molecular pump 9,the substrate 10 on the heater 12 is transported from the reactionchamber 11 to the first pre-stage chamber 1 through the second pre-stagechamber 2 with the gate valves 3 and 4 opened. Then, with the gate valve4 closed and the gate valve 5 opened, the pressure in the firstpre-stage chamber 1 is kept at a prescribed negative pressure by thecriosorption pump 6. The negative pressure may be less than 10⁻³ torr,preferably 10⁻⁶ to 10⁻⁹ torr. In the pre-stage chamber 1, thesemiconductor 10 is in atmosphere at less than 50° C. so that it doesnot experience thermal annealing, and is irradiated with light withoutmaking contact with air. By such a light irradiation, dangling bonds onthe semiconductor are made appear and thereafter neutralized by theneutralizing agent such as fluoride, chloride, oxygen, nitrogen, argon,krypton, xenon, helium or hydrogen and then the semiconductorfablication is now completed. In case that fluoride introducing isdesired as neutralization, use of fluoride gas of purity 99% or higheris suitable as an agent.

Namely, fluoride (m.p. -223° C., v.p. -187° C.) retained in a vessel isliquefied by liquefied nitrogen. Then, the liquefied fluoride isvaporized under negative pressure and purified as a very highly purifiedfluoride. In virtue of this processing, the fluoride is estimated tohave higher than 99.99% purity with very few oxygen having a dew pointlower than -60° C.

Thus introduced fluoride permeates into the surface, caves, gaps or thelike and neutralizes the dangling bonds of the semiconductor made appearby the photo annealing. In addition to replacing Si-F bindings fordangling bonds Si-, the fluoride replaces Si-F bindings also for Si-Hbindings which have relatively weak binding force.

FIG. 2 showes the semiconductor layer 26 on the substrate 10 fabricatedas in the above. The substrate 10 is made of a synthetic quartz. Thesemiconductor layer 26 is made of an nonmonocrystal amorphous silicondoped with hydrogen or halogen.

For the semiconductor layer 10, light irradiation annealing and thermalannealing are repeated for examining the change of the conductivity ofthe semiconductor layer 10. The photo annealing is carried out withhalogen light (100 mW/cm²) and the thermal annealing is carried outsupplying power from the heater. The measuring of the conductivity isaccomplished by means of the pair of proves 17 and 17' respectively incontact with the pair of electrodes 24 and 24' in situ, namely at anegative pressure.

In advance of describing the experimental result of measuring theconductivity of a semiconductor formed by the device according to theinvention, a brief explanation of prior art will be made for reference.

FIG. 3 showes the trend of the conductivity of a conventionalsemiconductor undergoing thermal annealing and photo annealing in turnin atmosphric circumference. As the conventional semiconductor a siliconsemiconductoe layer of 0.6μ thickness formed on a quartz glass pane wasmeasured.

In the following, the conductivity in presence of light (from a xenonlamp) is referred to as a photo conductivity and the conductivity inabsence of light is referred to as a dark conductivity.

In the figure, the initial photo conductivity is designated as 28-1, andthe initial dark conductivity as 28'-1. Light of AM1 (100 mW/cm²) isradiated from a xenon lamp to the semiconductor layer and as a resultphoto and dark conductivities 29-2 and 29'-2 were measured showingdecreases from the initial levels. Then, thermal annealing is carriedout, for this layer, at 150° C. for 2 hours, and thereafter conductivityis measured, the result being seen in the figure with 28-3 and 28'-3showing increases from the preceding levels. In this manner, thermal andphoto annealings are repeated one after another. As a result,Staebler-Wronski effect is demonstrates as shown in FIG. 3, in whichboth the photo and dark conductivities are decreased by photo annealingand recovers by virtue of thermal annealing with repeateadly.

Referring now to FIGS. 4 through 8 the transition of the conductivity ofthe semiconductor manufactured by the device of the invention is shownin response to optical and thermal annealings carried out one afteranother as in the above. On the curves, measurement results aredesignated by 29-1, 29-2, indicating the ordinal number of measurementand the apostrophe indicating the absence of light as like in FIG. 3.

FIG. 4 is a graphcal diagram showing the conductivity of a semiconductorformed by the device of the invention without treatment with theneutralizing agent. As described above, the measurings were made in thepre-stage chamber 1 under negative pressure without making contact withair.

On initial measuring, a dark conductivity 29-1, 1.5×10⁻⁸ Scm⁻¹ and aphoto conductivity 29'-1, 9×10⁻⁵ Scm⁻¹ are obtained at 25° C., and4×10⁻⁸ torr. Thereafter the semiconductor underwent photo annealing fortwo hours at 100 mW/cm² by a xenon lamp. A dark conductivity 29-2,6×10⁻⁹ Scm⁻¹, and a photo conductivity 29'-2, 3.5×10⁻⁵ Scm⁻¹ areobtained indicating decreases from the initial levels like the priorart. Next, the semiconductor underwent thermal anealing for 3 hours at150° C. Unexpectedly, as a result, the conductivity further decreasedunlike conventional measuring. This phenomenon discovered by theinventors has been named SEL effect. Meanwhile, although "SEL"corresponds to the abreviation of the assignee's name, this SEL wasderived rather from "State Exited by Light". The reason why SEL effecttakes place instead of Staebler-Wronski effect is supposed becausedangling bonds of the semiconductor are remain as they are duringrepetition in the evacuated chamber.

FIG. 5 showes a conductivity of the semiconductor measured in situ inthe device on the invention in which oxygen was introduced at 4×10⁴ Pa,approximately same as the partial pressure of oxygen in air, into thefirst pre-stage chamber as a neutralizing agent. 30-1 and 30'-1designate the photo and dark conductivities respectively before theintroduction of oxygen. 30-2 and 30'-2 designate the photo and thermalconductivities respectively after the introduction. After themeasurement 30-2 and 30'-2, the semiconductor is irradiated with lightfrom the halogen lamp at 100 W/cm² for two hours. As a result theconductivities 30-3 and 30'-3 were recognized to remain roughly as theywere, although a slight decrease was observed. Next, after thermalannealing for 3 hours at 150° C., the conductivity slightly recovered to30-4, 1.3×10⁻⁵ Scm⁻¹, and 30'-4, 1.2×10⁻⁹ Scm⁻¹. Further, thissemiconductor was left at negative pressure for a week. And, theconductivity was measured again and substantial increase was evaluated.Although the inventors repeated optical annealing for 2 hours andthermal annealing for 3 hours at 150° C. one after another, apprecablechange was not observed in the conductivity as shown with 30-6, 2.5×10⁻⁵; 30'-6, 3×10⁻⁹ ; 30-7, 2.7×10⁻⁵ ; 30'-7, 2.3×10⁻⁹. This trend can beexplained as infra.

Namely, after sufficiently producing dangling bonds as recombinationcenters by virtue of light irradiation under negative pressure, thedangling bonds are neutralized by oxygen or other neutralizer introducedand the adverse recombination centers are largely decreased which impairstabilization of the semiconductor. Once neutralized, the dangling bondscan not recover, even with photo annealing.

Further the semiconductor thus measured was taken out of the device andsubjected to measuring in the same manner as the above in theatmospheric circumference. However no Staebler-Wronski effect wasdemonstrated (30-8, and 30'-8).

Consequently, it was proved that with the device according to theinvention can be manufactured very stable semiconductor devices.

FIG. 6 showes another example of the transition of the conductivity of asemiconductor neutralized by a neutralizer agent.

In this example, before measurement the semiconductor was irradiatedwith light for 48 hours or at least more than 3 hours so that sufficientnumber of dangling bonds appeared, and highly prurified fluoride gas wasintroduced into the pre-stage chamber in order to neutralize thedangling bonds according to the fomula

    2Si-+F.sub.2 →2SiF.

The Si-F binding is expected stable even in the atmospheric air. Theelectronegativity of fluoride is 4.0 while the electronegativity ofoxide is 3.5. Thus, the repetation of photo and thermal annealings werecarried out for the semiconductor in the same condition as describedabove with FIG. 3. As seen from FIG. 6, the conductivity substantiallydid not fluctuate. From this experiment few recombination center appearsanew according on the semiconductor.

Referring to FIG. 7, the conductivity of further embodiment is shown. Inthis example, measuring results 29-1 to 29-5, and 29'-1 to 29'-5 areobtained in the same condition and the same specimen as in the FIG. 4.After measuring 29-5, and 29'-5, argon gas was introduced into thepre-stage chamber and rendered connected to dangling bonds which resideon the surface, caves or holes of the semiconductor. Then, as describedabove, SEL effect was observed in atomospheric pressure and in lowpressure (10⁻² to 10⁻⁶ torr). Thereafter, the semiconductor was treatedwith a thermal annealing at 100° to 500° C. in an ambience of the argonexcited by ultraviolet light. The conductivity of the semiconductor thustreated is shown by 32-1 and 32'-1. For this semiconductor, therepetition of photo and thermal annealings were carried out in turn asforegoing embodiment. As shown in FIG. 8, fluctuation byStaebler-Wronski effect was largely limited in comparison with priorart.

In the further embodiment, krypton, xenon, helium, hydrogen or blendsthereof are effectively used instead of argon and, in addition,existence of a trace quantity of sodium is very advantageous forstability of the characteristic of the semiconductor.

The recombination center density in a semiconductor according to theinvention is estimated less than 1×10¹⁷ to 18 cm⁻³, in some cases, lessthan 5×10¹⁶.

Though the above invention has been described with respect to specificpreferred embodiments thereof, many variations and modifications can beapplied as follow.

The neutralization process can be carried out in another chamberprepared separate from the manufacturing device.

After being irradiated, the semiconductor can be thermally processed at100° to 500° C., more desirably 250° to 300° C., at ambient pressure inthe atmosphere of fluoride mixture made active by ultraviolet lightradiation. For this semiconductor thus processed, almost samecharacteristic as shown in FIG. 6 was observed.

The present invention is also applicable to semiconductor layers whichare fabricated by photo CVD method.

Besides amorphous silicon, SixC_(1-x) (0<X<1), SixSn_(1-X) (0<X<1),SixGe_(1-X) (0<X<1), in which hydrogen and/or fluoride is doped, oramorphous silicon fluoride or other nonmonocrystalline semiconductor canbe applied the invention for.

As neutralizer agent, fluoride such as HF, CHF₃, CH₂ F₂, CF₄, GeF₄, Si₂F₆ or so on, and chloride such as HCl, CHCl₃, CH₂ Cl₂, CCl₂ F₂, or so oncan be employed with irradiation of ultraviolet light. Also oxide can beused in cooperation with ultraviolet light which decomposes the oxideinto atoms.

As easily understood, additives utilized in the invention as neutralizerare entirely different from that of prior art, for example, described inU.S. Pat. No. 4,226,898 in which these additives are introducedsimultaneously with fabricating process of a semiconductor carring outin the ambience of reactive gas including impurities.

What is claimed is:
 1. In a method of preparing non-single-crystallinesemiconductor films, the steps comprising:forming anon-single-crystalline semiconductor film including silicon and hydrogenon a substrate by plasma or photo CVD deposition of a gaseous compoundof silicon and hydrogen; effecting, under a vacuum, photo-annealing ofsaid semiconductor film in order to regenerate dangling bonds bybreaking the bonds between the silicon and hydrogen under said vacuum;introducing a neutralizing agent selected from the group consisting ofoxygen, fluorine, chlorine, and nitrogen after producing said danglingbonds to terminate the dangling bonds with the neutralizing agent.
 2. Amethod of claim 1 wherein said agent is chosen out of halogen, inertgas, oxygen or nitrogen.
 3. A method of claim 2 wherein said photoannealing is carried out under less than 10⁻³ torr.
 4. A method of claim3 wherein said photo annealing is carried out at less than 50° C.